Instantaneous frequency is defined, like angular velocity in rotational physics, as the derivative of the instantaneous phase (or angle). An alternative frequency measure is the change in instantaneous phase over a time interval divided by that time interval. This is equal to the average value of instantaneous frequency in the interval, which can be a sample period. It is arguably better than instantaneous frequency and is faster to compute. Spikes in instantaneous frequency are huge transient values, which, though they appear unrealistic, are a consequence of the definition of instantaneous frequency. The magnitude of instantaneous frequency spikes can exceed Nyquist frequency; the corresponding average instantaneous frequency values cannot. The decrease in the frequency content of seismic energy that has passed through a lowQ body, such as a gas reservoir, will likely be small and the resulting low frequency shadow on an instantaneous frequency display will be difficult to observe. Yet instantaneous frequency has great value in data analysis; in an example, it unmasked an anomaly and showed it to be an error in data processing.